1. Field of the Invention (Technical Field)
The present invention relates to rotor blades, particularly wind turbine rotor blades, and specifically to an aeroelasticly tailored turbine blades.
2. Background Art
Whenever the blades on a wind turbine are twisted, the twist directly influences the blade""s angle of attack, thereby changing loads and affecting output power. Classic pitch control used in not only wind turbines, but in rotors of all types, directly exploits these principles. When the pitch changes are sufficiently rapid, they can affect not only average rotor loads and turbine power, but vibratory loads as well, influencing fatigue life throughout the system. Even quite small angles of twist can have significant impact.
The general concept of rotor blades that passively adapt to the incident wind loading is not new. Mechanisms that adjusted blade angle of attack in response to the thrust loading were quite popular in the early days of the modem wind energy push of the late twentieth century. Approaches and objectives were quite varied. One effort regulated power with a centrifugally loaded mass on an elastic arm. Cheney, M. C. and Speirings, P. S. M. (1978) xe2x80x9cSelf Regulating Composite Bearingless Wind Turbine,xe2x80x9d Solar Energy, Vol. 20. Another attempt employed a system for cyclically adjusting pitch for per rev load balancing. Bottrell, G. W. (1981) xe2x80x9cPassive Cyclic Pitch Control for Horizontal Axis Wind Turbines,xe2x80x9d Proceedings of Wind Turbine Dynamics, NASA Conf. Pub. 2185, DOE Pub. CONF-810226, Cleveland, Ohio. The North Wind 4KW had a system for passively adjusting pitch for both power and load control. Currin, H. (1981) xe2x80x9cNorth Wind 4kW xe2x80x98Passivexe2x80x99 Control System Design,xe2x80x9d Proc. Wind Turbine Dynamics, NASA Pub. 2185, DOE Pub. CONF-810226, Cleveland, Ohio. Others have studied alleviating yaw loads with cyclic pitch adjustments. Hohenemser, K. H. and Swift, A. H. P. (1981) xe2x80x9cDynamics of an Experimental Two Bladed Horizontal Axis Wind Turbine with Blade Cyclic Pitch Variation,xe2x80x9d Wind Turbine Dynamics, NASA Pub. 2185, DOE Pub. CONF-810226, Cleveland, Ohio.
Also, a Garrad-Hassan report, for example, evaluated the use of all available blade loads to effect pitch changes that would regulate the power output of a turbine, aiming at a flat power curve in high winds. Corbet, D. C. and Morgan, C. A. (1992) xe2x80x9cReport on the Passive Control of Horizontal Axis Wind Turbines,xe2x80x9d ETSU WN 6043, Garrad Hassan and Partners, Bristol, UK, But only pitching to feather was evaluated to avoid the vagaries of predicting power output in the post-stall regime. The conclusion was that perfect regulation is very difficult to achieve, and that even less than perfect regulation is a challenge.
Regarding the construction of passively adaptive blades, Karaolis et al. introduced the concept of using biased lay-ups in blade skins on the surface of blades to achieve different types of twist coupling for wind turbine applications. By changing the blade skin surface from an orthotropic to a biased fiber lay-up, the blade can be aeroelastically tailored with minimal disturbance to the beam stiffness properties or manufacturing costs. Karaolis suggested that in addition to using the flapwise or centrifugal loading to twist a blade, it might be useful to internally pressurize a spar and use changes in the pressure to actively control the angle of blade twist. Karaolis, N. M., Mussgrove, P. J., and Jeronimidis, G. (1988) xe2x80x9cActive and Passive Aeroelastic Power Control using Asymmetric Fibre Reinforced Laminates for Wind Turbine Blades,xe2x80x9d Proc. 10th British Wind Energy Conf., D. J. Milbrow Ed., London, March 22-24, 1988; Karaolis, N. M., Jeronimidis, G., and Mussgrove, P. J. (1989) xe2x80x9cComposite Wind Turbine Blades: Coupling Effects and Rotor Aerodynamic Performance,xe2x80x9d Proc., EWEC""89, European Wind Energy Conf., Glasgow, Scotland, 1989. FIG. 1.1 from the prior art shows how different fiber orientations in a blade skin can be used to acheive bend-twist or stretch-twist coupling. In his 1988 report, Karaolis mapped out the combinations of two direction lay-ups to maintain strength and produce twist coupling in an airfoil shape.
Middleton et al. and Infield et al.designed, analysed, fabricated and tested a xe2x80x9cstretch- twist coupledxe2x80x9d blade developed to control the rotor in a runaway scenario. Their composite blade was fabricated using a helical layup with layers of glass and carbon fibers. Measured twist coupling agreed well with predictions and measured runaway speeds were actually less than predicted. Middleton, V., Fitches, P. Jeronimidis, G. and Feuchtwang, J. (1998) xe2x80x9cPassive Blade Pitching for Overspeed Control of an HAWT,xe2x80x9d Wind Energy 1998, Proceedings of the 20th British Wind Energy Association Conference, Cardiff University of Wales, Sep. 2-4, 1998; Infield, D. G., Feuchtwang, J. B. and Fitches, P. (1999) xe2x80x9cDevelopment and Testing of a Novel Self-Twisting Wind Turbine Rotor,xe2x80x9d Proceedings of the 1999 European Wind Energy Conference, pp 329-332, Nice, France, Mar. 1-5, 1999.
Another report on aeroelastic tailoring concluded that the use of aeroelastic tailoring of the Fibre Reinforced Plastics to control limited torsional deformation is a promising way to improve rotor blade design. Kooijman evaluated building the elastic coupling into the blade skin. Some of his conclusions for blades designed for the xe2x80x9cSmart Rotorxe2x80x9d were that: (1) Bending-twist coupling gives the potential for a few percentages of energy yield improvement for constant-speed pitch-controlled turbines and improves starting torque by 10%; (2) Optimal constant-speed pitch-controlled rotor production is obtained with the inboard span twisting to feather and the outboard 60% of the span twisting toward stall as wind speed increases; (3) The coupling is best achieved with hybrid carbon/glass reinforcement in the cross ply direction; and (4) Bending-torsion flexibility is about 10% less than a standard construction. Kooijman, H. J. T. (1996) xe2x80x9cBending-Torsion Coupling of a Wind Turbine Rotor Blade,xe2x80x9d ECN-I 96-060, Netherlands Energy Research Foundation ECN, Petten the Netherlands.
For constant speed rotors, enhanced stall control of wind turbines has been used in the past to improve the energy capture of rotors by allowing the rotor size to grow while maintaining a low maximum rating on other components in the system. Families of airfoils have been published that have since been used to stall regulate turbines at lower power levels with the associated reduced system cost of energy. Tangler, J. and Somers, D. (1995) xe2x80x9cNREL Airfoil Families for HAWTs,xe2x80x9d Proc. Windpower ""95, American Wind Energy Association, Washington D.C.; Klimas, P. C. (1984) xe2x80x9cTailored Airfoils for Vertical Axis Wind Turbines,xe2x80x9d SAND84-1062, Sandia National Laboratories, Albuquerque, N.Mex. An aeroelastically tailored blade that twists to stall in response to flap loads has a similar effect.
We have examined previously the generic coupling effects on annual energy production of a nominally 26 meter diameter stall regulated wind turbine. The blades were assumed to twist to stall, reducing maximum power. The rotor diameter was then increased to bring maximum power back up to its initial level. Twist distributions were specified by prescribing a maximum tip amplitude and a spanwise variation, varying with wind speed in either a linear or quadratic fashion. A twist proportional to power was also used. It was discovered that the details of spanwise variation or how the twist varied with wind speed (or power) had only minor impacts. The twist-coupled blades combined with larger rotors increase power in the important middle-range of wind speeds while power in high winds remains the same. Studies which investigated the increase in annual energy as a function of the annual average wind speed showed that for a maximum twist angle of one degree the energy capture is increased by about 5% and for two degrees, about 10%. The improvements are not overly sensitive to the wind resource. Lobitz, D. W., Veers, P. S., and Migliore, P. G. (1996) xe2x80x9cEnhanced Performance of HAWTs Using Adaptive Blades,xe2x80x9d Proc. Wind Energy ""96, ASME Wind Energy Symposium, Houston Tex., Jan. 29 -Feb. 2, 1996, the disclosure of which is incorporated herein by reference.
Whenever the wind turbine blade becomes aeroelastically xe2x80x9cactive,xe2x80x9d that is, the elastic deformations play a role in the aerodynamic loading, dynamic stability will be affected. We previously have addressed two of the most common stability constraints, namely classical flutter and divergence. Classical flutter is the condition where the phasing between the aerodynamic load fluctuations and elastic deformations are such that a resonant condition is achieved. Every wing will have a flutter boundary at some speed; for wind turbines the boundary is defined at the rotational speed (typically determined in still air) at which the blade will flutter. The stability margin is the difference between the flutter speed and normal operating speed. Divergence is a quasi-static condition where the blade twists in response to increasing load in a direction that further increases the load. If this condition exists on a blade there will be an operating speed at which the increase in loads caused by the deformation exceeds the ability of the blade to resist the load, called divergence. Lobitz, D. W. and Veers, P. S. xe2x80x9cAeroelastic Behavior of Twist-Coupled HAWT Blades,xe2x80x9d AIAA-98-0029, Proc. 1998 ASME Wind Energy Symposium held at 36th AIAA Aerospace Sciences Meeting and Exhibition, Reno, Nev., Jan. 12-15, 1998, the disclosure of which is specifically incorporated herein by reference.
The stability boundaries were determined with respect to the amount of twist coupling built into the blade. A coupling coefficient, xcex1, which varies between -xe2x88x92and 1, was defined to facilitate the generic examination of stability effects. For a blade with bending-twist coupling, and prescribed bending and twisting stiffnesses, this coefficient represents a range for the coupling stiffness wherein the system remains positive definite.
Creating a bending twist/coupled finite element model of the 10 meter diameter NREL Combined Experiment Blade for use in the MSC NASTRAN commercial software, the flutter and divergence stability boundaries were mapped over the range of possible xcex1""s. Instability occurs when the design operating speed exceeds these boundaries. Results indicate that the stability boundaries are not exceeded even with coupling levels up to 80% of the theoretical maximum, although the stability margins decrease toward the extreme values of the coupling coefficient. As xcex1 becomes increasingly positive, the blade bends and twists toward stall, increasing angle of attack and also loads, and this reduces the divergence stability margin. Conversely, as xcex1 becomes increasingly negative, the blade twists toward feather reducing aeroloads and increasing the divergence stability margin. However, in this region, the classical flutter stability margin decreases, although not as severely as the divergence margin for positive xcex1""s. Lobitz, D. W. and Veers, P. S. (1998) xe2x80x9cAeroelastic Behavior of Twist-Coupled HAWT Blades,xe2x80x9d AIAA-980029, Proc. 1998 ASME Wind Energy Symposium held at 36th AIAA Aerospace Sciences Meeting and Exhibition, Reno, Nev., Jan. 12-15, 1998.
Twisting to feather in response to increasing winds is a known potential means to reduce the dynamic loading on the blades, and hence the rest of the system. Load reductions have been demonstrated by linking a pitch control system to flapwise blade loads using simple integral control. Eggers, A. J. Jr., Ashley, H., Rock, S. M., Chaney, K., and Digumarthi, R. (1996) xe2x80x9cEffects of Blade Bending on Aerodynamic Control of Fluctuating Loads on Teetered HAWT Rotors,xe2x80x9d J. of Solar Energy Engr., Vol. 118, No. 4, November 1996. Eggers"" results indicated that the rms blade bending response to turbulent winds could be reduced by about half. And this was accomplished with rms pitching angles of 3 degrees if ⅓ span ailerons are used and substantially less with full-span pitch control. The fatigue implications of such a substantial decrease in cyclic loading are enormous, measured in increased lifetime or reduced blade weight.
An aeroelastically twisting blade is similar to the control system investigated by Eggers in that the blade pitch angle responds to bending loads in a way similar to a proportional, rather than an integral, controller. The ability of bending-twist coupled blades to attenuate (or exacerbate) the cyclic loading was previously investigated for a 33 meter diameter rotor employing three different control strategies; constant speed stall-controlled, variable speed stall- controlled and variable speed pitch-controlled. Transient structural dynamic simulations of the bending twist/coupled rotor were carried out using the ADAMS commercial software modified to include twist/coupling. Ensembles of 10 minute turbulent wind histories were generated with the SNLWIND-3D software and used to drive the rotor. Average windspeeds of 8, 14 and 20 m/s (peak power for stall-control occurs near 17 m/s and for pitch-control near 12 m/s) were investigated for both high and low turbulence intensities. Material damage exponents of 3, 6 and 9 were used for computing damage. Lobitz, D. W. and Laino, D. J. xe2x80x9cLoad Mitigation with Twist-Coupled HAWT Blades,xe2x80x9d Proc. 1999 ASME Wind Energy Symposium held at 37th AIAA Aerospace Sciences Meeting and Exhibition, Reno, NV, Jan. 11-14, 1999, the entire disclosure of which is hereby incorporated by reference.
Results for the constant speed stall-controlled case indicated that twist/coupling toward stall produces significant increases in fatigue damage, and for a range of wind speeds in the stall regime apparent stall flutter behavior is observed. For twist-coupling toward feather with a coupling coefficient of magnitude, 0.6, fatigue damage is decreased by 20 to 70% with the higher percentages occurring at the lower average windspeeds. Concurrent with lower fatigue damage estimates for twist-coupling toward feather, maximum loads decreased slightly, especially for the lower average windspeeds. For the case where the pitch offset is altered to bring the power curve of the twist coupled rotor into better agreement with that of the uncoupled one, differences in average power are minimal. Lobitz, D. W. and Laino, D. J. xe2x80x9cLoad Mitigation with Twist-Coupled HAWT Blades,xe2x80x9d Proc. 1999 ASME Wind Energy Symposium held at 37th AIAA Aerospace Sciences Meeting and Exhibition, Reno, Nev., Jan. 11-14, 1999.
There are limits to the amount of coupling that can be achieved with asymmetric fiber lay-ups. The best direction and the maximum coupling are a function of the fiber and matrix properties. Previous efforts have indicated that stiffer fiber materials result in the higher coupling coefficients, with maximum xcex1 for flat plates just below 0.8 for a graphite-epoxy system and 0.6 for a glass-epoxy system. The carbon system achieves maximum coupling with all the fibers at about 20 degrees to the axis of bending while the glass maximum is at about 25 degrees. Tsai, S. and Ong, C-H. (1998), xe2x80x9cD-Spar Blade Design and Manufacture,xe2x80x9d unpublished contractor reports, Sandia National Laboratories contract BB-6066 Stanford University. Composite, uniform, D-spars have been designed and fabricated that possess coupling coefficients in the range of 0.6. Ong, C. H. and Tsai, S. W. xe2x80x9cDesign, Manufacture and Testing of a Bend- Twist D-Spar,xe2x80x9d Proceedings of the 1999 ASME Wind Energy Symposium, Reno, Jan. 11-14, 1999, the teaching of which are hereby incorporated by reference.
There have been publicized efforts to develop a special twist/axial coupled spar that will rotate a tip mechanism through large enough angles to control power and provide some over. speed protection. Joose, P. A. and van den Berg, R. M. xe2x80x9cDevelopment of a TenTorTube for Blade Tip Mechanisms,xe2x80x9d P7.17, Proc., 1996 European Union Wind Energy Conf. and Exhib., Gxc3x6tegorg, May 20-24, 1996; van den Berg, R. M., P. A. Joosse, and B. J. C. Visser xe2x80x9cPassive Power Control by Self Twisting Blades,xe2x80x9d Proc., European Wind Energy Association Conf. and Exhib., Thessaloniki, Oct. 10-14, 1994.
There also have been showings of how small turbines can have improved speed regulation with twist/axial coupling. Infield D. G. and J. B. Feuchtwang xe2x80x9cDesign criteria for passive pitch control of wind turbines using self-twisting blades,xe2x80x9d International Journal of Ambient Energy, Vol. 16, No. 3, July 1995; Feuchtwang, J. B. and D. G. Infield xe2x80x9cAerofoil profile section for passive pitch control using self-twisting blades,xe2x80x9d Wind Energy Conversion, Proc. 17th BWEA Wind Energy Conf., ed. J. Halliday, BWEA, Warwick, Jul. 19-21, 1995. A common feature in these works is to provide relatively large rotations to achieve substantial amounts of power regulation. Most have used stretch twist coupling on variable speed systems to assist in over-speed control or power regulation and rely on large angles of twist to accomplish complete control of high wind loads.
And it has been demonstrated that even with relatively small twists (that incidentally also enhance regulation), a stall controlled, fixed pitch system could be operated with a larger rotor to achieve net energy enhancements without increasing the maximum power rating. Lobitz, D. W., Veers, P. S., and Migliore, P. G. xe2x80x9cEnhanced Performance of HAWTs Using Adaptive Blades,xe2x80x9d Proc. Wind Energy ""96, ASME Wind Energy Symposium, Houston, Jan. 29 -Feb. 2, 1996.
The reorientation of the fiber directions in the rotor blade surface skin or spar to achieve either flap-load or extension-load coupling with blade twist, thus has potential to be a cost effective and reliable aeroelastic tailoring approach. The present invention involves modest blade rotations produced by elastic twist coupling of the blade as it bends or extends without any additional mechanisms or devices. There are a number of possible uses of aeroelastic tailoring in wind turbine applications. They include stall enhancement to permit larger diameter rotors for improved average energy capture, dynamic effects including stability issues, and load alleviation through twist coupling toward feather. In the present invention, the use of elastic twist coupling in the rotor blade is exploited to alleviate loads to promote structural longevity.
The present invention exploits modest twist angles to produce load alleviation and perhaps power regulation or enhancement through bend/twist coupling. The manufacturing process will depend on the type of coupling to be produced. Fiber winding is well suited to producing stretch-twist coupling in a spar, while clam-shell construction with the top and bottom skins manufactured separately is best suited to bend-twist coupling.